Linear low density polyethylene polymers (LLDPE) polymerized by using conventional Ziegler catalysts are used as various articles, such as films, sheets, blow-molded articles, injection-molded articles and the like, because of their excellent moldability, transparency, strength and heat-seal strength. In the field of films and sheets, they are widely used as various wrapping materials (as disclosed in JP-A-52-135386 and JP-A-61-284439). (The term "JP-A" used herein means an "unexamined published Japanese patent application".) Recently, however, higher transparency and film strength have been demanded. Further, films adapted to be processed by an automatic bag making machine have needed to be processed at a higher speed. Thus, films having excellent low temperature heat-seal properties have been demanded.
When a film is heat-sealed at a high speed, it must be processed in a short period of time that tends to cause the film to be fused insufficiently. The resulting product is liable to shortage of strength. In order to eliminate this difficulty, a method may be employed which comprises the rise in the temperature of the heat seal bar. However, this method is disadvantageous in that the laminated film is curled. Another method is to lower the resin density and hence to lower the melting point of the resin. However, conventional linear low density polyethylene (LLDPE) is disadvantageous in that when the density thereof are lowered, it raises the content of highly branched low molecular components which are partly eluted on the surface thereof, rendering the film sticky and less peelable. The eluate also can migrate into the content of the bag.
Further, LLDPE films have wider application. For example, LLDPE films can be laminated with a resin having excellent gas barrier properties such as polyamide, polyester and saponification product of ethylene-vinyl acetate or a resin having a high rigidity such as high density polyethylene and polypropylene, to form a laminate having good heat-seal properties as well as high gas barrier properties or high nerve, which can be used as a bag-forming material adapted for high speed processing, such as a food wrapping material and bag, a food container and a medicine container.
The LLDPE films have been widely used as heat-sealing films (sealant film) for these laminates. These laminates may be laminated on a base material (base film) by extrusion lamination process, dry lamination process, sand lamination process, co-extrusion T-die process, co-extrusion blown film process or the like. Among these lamination processes, the dry lamination process is conducted with a polyether adhesive, a polyurethane adhesive, a vinyl acetate adhesive, an isocyanate adhesive, a polyethyleneimine adhesive or the like. Some of these adhesives are subjected to heat treatment after lamination to undergo curing. During this heat treatment process, the lubricant incorporated in the film can migrate to the adhesive layer, problems arise as rendering the surface of the laminate less peelable and less lubricating and hence causing troubles in the bag making process and filling process.
In recent years, in order to meet the above requirements and remove the disadvantages, a high strength ethylene-.alpha.-olefin copolymer having a very narrow molecular weight distribution and a very narrow composition distribution has been developed by employing a preparation process in the presence of a metalocene catalyst. However, such an ethylene-.alpha.-olefin copolymer has some disadvantages. Such an ethylene-.alpha.-olefin copolymer has a very narrow composition distribution and thus shows a very sudden change of viscosity and modulus with temperature. Thus, the applicable temperature and extrusion conditions under which such an ethylene-.alpha.-olefin copolymer is molded is restricted making it difficult to mold such an ethylene-.alpha.-olefin copolymer. Further, such an ethylene-.alpha.-olefin copolymer is disadvantageous in that it gives a molded product which can exhibit a sufficient heat resistance, a proper heat-seal strength or a good hot tack strength only in a narrow temperature range. For example, when applied to the field of film, sheet or the like, such an ethylene-.alpha.-olefin copolymer can be easily heat-sealed. Accordingly, such an ethylene-.alpha.-olefin copolymer is often heat-sealed and used in the form of bag. In this application, sufficient hot tack properties are required. In other words, when such a bag is filled with a content, the seal area which has been heat-sealed is immediately pulled under load and may be peeled. In this application, a heat-sealable ethylene-.alpha.-olefin copolymer which can be heat-sealed in a wide temperature range to show a high resistance to peeling shortly after sealing is desirable.
As means for improving moldability of such an ethylene-.alpha.-olefin copolymer having a very narrow composition distribution, an attempt has been made to improve the melt properties of the resin while keeping the composition distribution narrow by using a metalocene catalyst having a plurality of ligands (as disclosed in JP-A-6-206939). However, this method is disadvantageous in that it gives a wider molecular weight distribution or produces long-chain branches, causing decrease in strength. Further, an attempt has been made to improve the moldability by using a mixed catalyst system of Ziegler catalysts (as disclosed in JP-A-6-157631). However, this method gives a wider molecular weight distribution that impairs the strength.
In the field of electrical insulating materials for high voltage power cable, there have heretofore been widely used a high pressure process low density polyethylene, crosslinked polyethylene, etc. because they have excellent electrical properties.
One of difficulties with high voltage power cable is power loss developed during power transmission. The reduction of power loss is an important demand to be met.
The reduction of power loss can be accomplished by enhancing the high voltage properties, particularly volume resistance, of the insulating material. However, the insulating material for power cable is heated to high temperatures (about 90.degree. C.) by Joule heat generated by the passage of current in the vicinity of the inner conductor but is kept at the ambient temperature (about 20.degree. C.) in the vicinity of the outer conductor. The conventional polyethylene shows a marked volume resistance drop with the rise in temperature. Accordingly, the polyethylene shows a marked volume resistance drop in the vicinity of the inner conductor through which current flows. Thus, an electric field is concentrated in the vicinity of the interface of the outer conductor with-the insulating material, lowering the breakdown strength of the insulating material. This phenomenon presents a great problem particularly with direct current power cables. Therefore, insulating materials having a small temperature dependence of volume resistance have been desired.
When a high pressure process low density polyethylene is used as an insulating material for high voltage cable, its low melting point gives poor electrical properties. On the other hand, the conventional low pressure process polyethylene, which exhibits a high melting point, does not necessarily have excellent electrical properties, probably due to the effect of catalyst residue. The conventional low pressure process polyethylene is also disadvantageous in that it exhibits a poor flexibility.
In order to improve the electrical properties of the low pressure process polyethylene, a method has been proposed which comprises grafting maleic anhydride onto the polyethylene (as disclosed in JP-A-2-10610). However, this method cannot necessarily give full solution to the foregoing problems, including flexibility.
As a high volume resistance material having excellent heat resistance and flexibility there have been disclosed an insulating material obtained by blending 100 parts by weight of a high pressure process low density polyethylene having a density of 0.92 g/cm.sup.3 with from 0.5 to 20 parts by weight of a linear low density polyethylene having a density of from 0.91 to 0.94 g/cm.sup.3 (as described in JP-A-5-26723). However, the foregoing composition leaves something to be desired because it has a great temperature dependence of volume resistance in the vicinity of the inner conductor.